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A GIS-based modeling of environmental health risks in populated areas of Port-au-prince, Haiti

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par Myrtho Joseph
University of Arizona - Master in Natural Resources Information System 1987

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2.4 Risk Assessment

A risk assessment is formally an estimation of the types and the degrees of danger posed by a hazard. It comprises three elements, which are: 1) hazard identification, 2) risk and vulnerability estimation, and 3) evaluation of the social consequences (Ferrier and Haque 2003). The general formula that arises from this conceptual approach is R = pxV, where R represents risk, p the probability of occurrence, and V, vulnerability to loss. This formula is substantiated and enhanced in the manual for policy makers and planners of the United Nations Agency regarding disaster mitigation, which in addition to hazard and vulnerability inserts element at risk (UNDRO 1991, Diley et al. 2005, BMRC 2006):

Risk = Hazard x Element at Risk x Vulnerability

Methodologies applied in risk determination include the stochastic and the systematic approach (Ferrier and Haque 2003, UNDP 2004, Dilley et al. 2005). The stochastic (or quantitative) method involves estimating the probability of occurrence and intensity of a hazard, based on historical data. One of the major weaknesses of this approach is the insufficient length of historical information, and even its non-existence in some areas (Huppert and Sparks 2006). In addition, for non-frequent events, some physical processes such as deforestation, urban sprawl, extension of impervious areas, and construction in high slopes may affect vulnerability of places to a certain hazard. Thus, projection of zones susceptible to disasters based solely on past occurrences may be misleading. The systematic or deterministic method depends on prior knowledge about the physical conditions and processes that control the chance of the occurrence of a hazard (El Morjani 2007). This type of information may be more accessible. An integrated approach was used by Diley et al. (2005) in a study sponsored by the World Bank. The application of either or both methodologies relies heavily on the information situation at hand. Given the strict limitation of historical and detailed spatial data regarding environmental health hazards we are dealing with for the study area, this paper relies mainly on the deterministic approach, which offers the advantage of being integrative, and does not necessitate factual and historical information.

2.5 Hazard identification or delineation

Hazards can be characterized by their event frequency and associated characteristics, their probability of exceeding a certain threshold, and their probability of occurrence based on a range of physical factors (UNDP 1994, Dilley et al. 2005). These concepts were previously generalized in Hansen (1984) and Hansen and Frank (1991) which classified hazards determination into two approaches: indirect/causal and direct/occurrence. The earlier is based on a priori knowledge of the underlining factors of hazards in the area under study and involves two sub-approaches: heuristic and statistical. In the heuristic approach factors are ranked and weighted based on their assumed importance in causing the hazards; in the statistical approach the role of each factor is determined in comparison to the observed relations with past/present distribution of the hazard. The fundamental principle of the direct/occurrence approach relies on the observed distribution of the hazards over time.

Whereas the advantages of these techniques are unquestionable, some drawbacks are data availability particularly for small areas, validation of the information, time consumption for data collection, and precision. Error in mapping can influence the predictive ability of the model that may not be possible to be extrapolated to other areas. But the most important limitations are spatial scale and availability of reliable historic data (Dilley et al. 2005). The intricate context of data collection at spatial and temporal scale in health hazards may make it even more inappropriate.

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